The diagonal leaves out the shape
Store labels measure from one panel corner to the opposite corner. That number says nothing about how the diagonal splits into width and height. Aspect ratio supplies the missing part.
This matters on a desk. A 34-inch 21:9 screen works out to about 31.3 inches wide and 13.4 inches tall. A 27-inch 16:9 panel is about 23.5 by 13.2 inches. The extra diagonal mostly travels sideways.
27-inch, 16:9
34-inch, 21:9
What showed up in our 27 vs 34 test
During our QA pass, the default comparison made the width jump obvious. The height result was the useful surprise: roughly 0.15 inches separated the two panels. On paper, seven diagonal inches sounds large. At the bottom edge of the drawing, both screens sit at almost the same height.
Switch to Overlay when shape is the concern. Side by side is easier for desk planning, and the table gives exact values when a drawing feels too close to call.
How the calculation works
The screen forms a right triangle. For an aspect ratio of w:h, the ratio diagonal is √(w² + h²). Physical width equals the screen diagonal multiplied by w, then divided by that ratio diagonal. Height uses h instead.
Area is width multiplied by height. When resolution is present, PPI equals the pixel diagonal divided by the physical diagonal.
Where the outline stops being exact
- Rounded corners can remove a little usable area from phones and tablets.
- Curved displays. Published diagonal follows the panel specification; desk depth and curve remain model-specific.
- Bezels vary. Check the manufacturer’s full product width before buying an arm, fitting a shelf, or placing speakers beside a monitor.
- The actual-size mode needs calibration on each display and browser zoom level.
Physical size and resolution answer different questions
A 32-inch 1080p screen and a 32-inch 4K screen occupy the same physical rectangle. The 4K panel packs four times as many pixels into it. Add both resolutions in the tool to compare PPI and megapixels without changing the outline.
Text size also depends on operating-system scaling and viewing distance. For a closer look at density, open the DPI and PPI test. A display’s motion is separate again; the refresh rate test measures frame timing in this browser.
Use actual size carefully
CSS pixels are logical layout units. Their physical size changes with the display, browser zoom and operating-system scaling. That is why an uncalibrated web page cannot promise a one-to-one physical drawing.
Our calibration outline uses the ID-1 card dimensions, 85.60 by 53.98 millimeters. Set browser zoom to your normal level, hold a bank card against the outline, then adjust the slider. Recalibrate after moving the browser to another monitor (easy to forget when a laptop is docked).
Privacy and saved comparisons
Screen entries and calculations stay in your browser. Copy Summary places plain text on your clipboard. Copy Comparison Link adds the current fields to the URL, which means anyone receiving that link can read the values you chose. No account is required.
Method reviewed August 2, 2026. See our editorial policy for how tools and supporting claims are checked.
FAQ
- How is screen width calculated from the diagonal?
- The tool treats the width, height and diagonal as a right triangle. It combines the diagonal with the selected aspect ratio, then uses the Pythagorean theorem to calculate viewable width and height.
- Does the comparison include the bezel or stand?
- No. Published screen size describes the viewable panel measured diagonally. Bezel thickness, stands, curved backs and phone bodies depend on the exact product model.
- Why can two screens with the same diagonal have different areas?
- Aspect ratio changes how the diagonal is divided between width and height. A wider screen gives up height, so equal diagonal labels can produce different panel areas and shapes.
- Is the actual-size view accurate on every device?
- Only after calibration. Browsers lay pages out in CSS pixels and do not promise that one CSS inch matches one physical inch. Match the card outline to an ID-1 bank card before using the actual-size view.
- Do you store the screen sizes I enter?
- No. The calculations run in this browser. A comparison is placed in the URL only when you choose Copy comparison link, so you can save or share that specific setup.
- What does adding resolution change?
- Resolution lets the tool calculate pixel density in PPI and total megapixels. It does not change the physical outline, which comes from diagonal size and aspect ratio.
Sources
- W3C CSSOM View Module: CSS pixels and screen information
- ISO/IEC 7810: ID-1 identification card dimensions
Links go to official browser docs or primary references when available.
Check the panel after choosing a size
Physical dimensions cannot reveal stuck pixels, uneven gray, light leakage, or touch dead zones. Run the dead pixel test or screen uniformity test on the display itself.